The present invention relates to a tuning fork with reduced half-natural-frequency spurious response.
It is well known that a tuning fork which consists of a pair of metal, vibratory arms extending from the end of a stem, when struck, emits a tone of fixed pitch. The tuning fork, when excited by the input signal, is caused to vibrate at its natural or fundamental frequency, and thus the frequency of the input signal is equal to the fundamental frequency. Therefore, such tuning forks are widely used to convert an electrical signal applied to the tuning fork into a signal having natural frequency of the tuning fork, and also, in the arrangement known as an electromechanical reed filter, for the purpose of passing the frequency component of an applied electrical signal corresponding to the natural frequency of the tuning fork.
In the electromechanical reed filter, the tuning fork is provided with a pair of electromechanical transducers such as piezoelectric elements, one for converting an exciting input signal in mechanical vibration of the tuning fork and the other for converting the vibration of the tuning fork into an electrical signal having the same frequency as the vibration of the fork.
In a conventional tuning fork, as shown in FIG. 1, piezoelectric elements 2a and 2b each made of barium titante (BaTiO.sub.3) or the like are provided, respectively, on or near the vibratory node of the vibratory reeds or vibrating pieces 1a and 1b of a tuning fork vibrator 1, and the stem or base portion 1c of the tuning fork vibrator 1 is secured to a base 4 by a support rod 3, which functions as a common terminal. The piezoelectric elements 2a and 2b are electrically connected to terminals 5 and 6 of the base 4, and the tuning fork vibrator 1 is covered by a case 7 which is secured to the base 4 to form a housing. The tuning fork vibrator 1 is driven by a signal applied between the terminal 5 and the support rod 3, and the output signal is obtained from between the terminal 6 and the support rod 3.
However, in recent years, it has been desired to miniaturize such electronic parts as tuning forks and the like for use in increasingly compact electronic appliances. As the tuning fork is made smaller, the ratio of weight of the tuning fork vibrator 1 to the weight of the entire tuning fork increases due to maintain the similar configuration thereof, while the fraction of the weight made up by the remaining portion, including the base 4 and the case 7 decreases. Also, the percentage of the area of the vibrating piece occupied by the piezoelectric element becomes larger.
As shown in FIG. 2, when a conventional miniaturized tuning fork vibrator 1 is oscillating stably in the direction shown by arrows S in such a manner that during one half cycle of the vibration the vibrating pieces are approaching each other while during the other half cycle the vibrating pieces are moving apart since the weight of the part of the unit excluding the tuning fork vibrator itself is relatively small compared with that of the tuning fork vibrator, the part of the unit other than the vibrator is put into vibration at the frequency of the tuning fork vibrator but 180.degree. out of phase with the vibration of the vibrating pieces 1a and 1b. arrows U and T showing the vibration directions of the tuning fork vibrator and of the remaining portion of the unit, respectively. In other words, during one half cycle of the vibration both vibrating pieces 1a and 1b of the tuning fork vibrator are moving in the direction of arrow U.sub.1, while the remaining portion thereof brings to vibrate in the opposite direction (arrow T.sub.1), and then during the other half cycle both vibrating pieces 1a and 1b of the tuning fork vibrator are moving in the other direction (arrow U.sub.2) while the remaining portion moves in the first direction (arrow T.sub.2), resulting in torsional vibration or twisting motion between the tuning fork vibrator and the remaining portion, and a so-called (1/2)f.sub.0 spurious response is generated at approximately one-half of the natural vibration frequency f.sub.0. This limits the possible miniaturization of the tuning fork to a certain extent. In one approach to solving this problem the piezoelectric elements 2a and 2b may be replaced respectively by a pair of piezoelectric elements 2a' and 2a", which are placed one above the other on the vibrating arm 1a, oriented parallel to the longitudinal direction of a vibrating piece 1a, are disposed on the vibrating piece and a pair of piezoelectric elements 2b' and 2b", which are placed one above the other on the vibrating arm 1b, oriented parallel to the longitudinal direction of a vibrating piece 1b, as shown in FIG. 3. The piezoelectric elements 2a" and 2b', and 2a' and 2b" are electrically connected to terminal 5 and elements 2a' and 2" to terminal 6. The respective (1/2) F.sub.0 spurious responses generated by the two pairs of piezoelectric elements 2a", 2b' and 2a', 2b", respectively interfere destructively with each other, so that the net spurious response is small. However, the construction of the piezoelectric elements is complicated, and even with this construction, therefore, small tuning forks are hard to make.
It is the chief object of the present invention to provide a tuning fork of the above type without the disadvantages inherent in the conventional design and substantially without the (1/2)f.sub.0 spurious response that is generated at approximately one-half the natural vibration frequency f.sub.0 in conventional tuning fork vibrators, without interfering with the natural vibration of the tuning fork vibrator.
Another object of the present invention is to provide a tuning fork equipped with a damper between the fork itself and the housing, to prevent noise that would otherwise be caused by an external mechanical shock such as dropping the tuning fork.
A further object of the present invention is to provide a tuning fork which is strong in construction, but without causing any deformation of the tuning fork vibrator or the mounting arrangement thereof.
A still further object of the present invention is to provide a compact, high-quality tuning fork that is that is easy to assemble.